The secret life of the riverbed

From a distance, it looks like an old Second World War aerial photograph. Heavy lines mark out the roads. Those boxes, one after another, are houses on an estate. The blurred grey stuff - they are trees. But this is not something from Photographic Intelligence - it is a photogram of ice. As you get closer, you can trace the giant stress cracks, the tiny tendrils and the swirling ebb and flow of the river Taw, now caught frozen until spring comes.

Susan Derges has been working with water for seven years. She captures its ebb and flow in photograms - one of the earliest forms of photography using a method of taking a picture that doesn't involve a camera or a lens. With photograms, light sensitive paper is placed under the subject and a flash exposes its image onto the paper. "There was so much baggage and theory with camera-based photography, I wanted to simplify it and make the connection between image and subject as close as possible," says Derges, who has been making photograms since 1981 when she captured sound waves by placing a sound generator under light sensitive paper on top of which was a scattering of powder. The image made showed the "very beautiful organic patterns" sound can make.

The idea to work with water came to Derges after she moved to Dartmoor. But prior to this, she had lived and worked in Tokyo for five years. She went there as an artist, a graduate of the Chelsea and then the Slade School of Art, but in Tokyo the pace of life "made my art seem very inappropriate". Derges' art then was abstract and very labour intensive but, funnily enough, looked not dissimilar to her "river prints" of today. Back from Japan, Derges set up again in London and continued her work capturing "liquid" processes such as sound and also mercury.

In the early Nineties, she moved to the West Country. "When I moved to Dartmoor, what I had been setting up in my studio was suddenly on my doorstep - rivers, water, constant flux and change." And frogspawn, which Derges happened upon one morning. With the sun shining onto the pond, the spawn made shadows on the pond bed. Derges took some spawn back to her studio and did a whole set of photogram prints chronicling the metamorphosis of the spawn into frogs, called Full Circle. The water prints soon followed.

"The first time I worked with water was using a brook near where I live. I thought it would be possible to just lie the paper in the water, but of course it floated to the surface," explains Derges. So she made light boxes out of sheets of aluminium, attached the light sensitive paper (Cibachrome, so it makes a positive negative) to the bottom sheet of aluminum with double-sided tape - the paper gets wet in the process - and then fixed a lid on top to protect the paper from light until exposure.

Then, after making intensive recces, Derges, either alone or with helpers (the prints are life-sized and some are over 20 foot long), goes to the water's edge, removes the lids and submerges the weighted paper. This must be done at night and thus effectively the environment becomes her dark room.

Once the paper is in place, Derges must work fairly quickly. When she feels the moment is right, the flash is let off and the image is made. The ripples of water, any leaves either floating by or overhanging (no fish since they will have long been spooked) and other river bed debris will have been captured.

Many things are fascinating about this process. Because the image is life-sized, you get a real sense of the force of the water; of being there. This is further reinforced since the image is taken under water. It is a unique view of the water. Derges is keen on this. It is "as close as you could get to lying under the water yourself". Because each stretch of river is unique, each print becomes like an identifying fingerprint but a unique one that can never be repeated. The phases of the moon also affect the final print. One, called Waterfall and taken on a full moon, is an intense aquamarine colour. With Shore Line, a giant 24ft by 3ft print taken during a half moon, the magenta edges of the incoming surf were created by the "interference" of street lights reflecting on the water.

When the river prints were first exhibited, they followed the river Taw from its source on north Dartmoor to the sea and with it, the cyclical change of seasons. Hence the ice print when parts of the river were frozen, to a full flowing spring river. Now, some prints have been added, some taken away as Derges' work constantly evolves.

From the 19 Sept to the 24 Oct will the very last chance to see the river and shore line prints (shown along with the frogspawn collection) in this country. After that, these magical, hypnotic prints will travel to the Fraenkel Gallery in San Francisco and then the James Dan Ziger Gallery in New York. The work going to America will have evolved again: this time showing a year in the life of not only the river Taw but also the trees, whose foliage - from barren to leafy - and roots, interact with it.

Derges' prints will probably never make it back here. With each exhibition, the prints are bought by collectors. Unique, serene and beautiful ("I always feel my photographic work is like painting with light"), it's funny to think of bits of a humble Dartmoor river hanging on walls all over the world.

But if you don't make it to the exhibition in Cornwall, the Victoria & Albert museum has bought the dazzling sky-blue and lilac Waterfall print which will be shown in the Canon Photography Gallery in an exhibition entitled Silver and Syrup. "Susan's pictures are breathtaking," says the V&A's assistant curator of photography, Charlotte Cotton, who chose the print. "She uses photography to display the quality of water which is not visible to the naked eye and creates a new and exciting vocabulary for the medium of photography whilst calling to mind the earliest photographic processes and motivations of the 1840s. I can vividly remember every exhibition of Susan's work that I have ever been to - I can remember how I felt in the presence of her images more than anything else."

Susan Derges' `River Taw' exhibition will be shown at the Newlyn Art Gallery, New Road, Newlyn, Penzance, Cornwall from 19 September - 24 October 1998. Tel: 01736 363715. The `Silver and Syrup' exhibition at the Canon Photography Gallery, Victoria & Albert Museum, Cromwell Road, London SW7 will run from 26 November 1998 to 12 April 1999